747 Fleet's Age at Issue During Flight 800 Hearing

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Dozens of Boeing 747's are flying far longer than the manufacturer anticipated, with only limited attention to the aging of their components, according to testimony at a hearing today into the crash of T.W.A. Flight 800. But making changes to the planes or even just inspecting them more intensively could damage wiring and thus increase risk, some experts said, casting doubts on what should be done next.

The first Boeing 747 was delivered 28 years ago this month. The planes were designed for 20 years of life, 20,000 flights and 60,000 hours, a Boeing witness said today. But 240 planes are more than 20 years old, 95 have logged more than 20,000 flights and 380 planes have flown more than 60,000 hours.

The Trans World Airlines plane that crashed had flown for about 90,000 hours, 50 percent longer than the original design, and was 25 years old, but had flown only about 18,000 flights.

''We maintain that with appropriate maintenance there is no specific life limit on the 747 airplanes,'' said Robert Vannoy, Boeing's chief of 747 fleet support. Boeing and the airlines have intensively inspected the oldest planes for signs of trouble and made fixes as needed, he said.

Mr. Vannoy was testifying in the fourth day of hearings held by the National Transportation Safety Board.

The hearings have not found the cause of the explosion that destroyed the plane off Long Island in July 1996, killing all 230 people on board, but have turned up numerous problems with wiring, which may or may not have played a role.

Today, for example, investigators said that wiring from the plane that crashed had traces of water-borne cleaning fluids, meaning that as the plane was cleaned, the wiring was dampened. And Boeing experts said that wiring insulation for the 747 was chosen not just to insulate, but also to minimize weight and volume, since the plane carries about 150 miles of wire.

Whether the wiring caused the spark, or whether the crash had anything to do with the plane's age, is not clear. Today, Mr. Vannoy tried to dispel the idea that there was any problem with flying old airplanes. He said that they are intensely inspected and that if any problem is found on one, mechanics quickly inspect others.

''All these efforts have been focused on assuring continued safety,'' he said. ''I want to stress that. They have not been focused on making the airplanes last longer.''

At the time the plane was designed, few aircraft flew more than 20 years because they became obsolete in that period.

In the 70's, the expectation was that planes like the 747 would be superseded by fleets of supersonic transports. But Boeing is still producing the 747.

Of the 1,048 Boeing 747's delivered by Nov. 30 of this year, all but 81 are still in service; of those 81, about a third have been scrapped, a third are ''parked,'' meaning that they could be returned to service if economic conditions made it worthwhile, and a third are ''hull losses,'' an industry term meaning heavily damaged or destroyed in accidents.

According to the Federal Aviation Administration, as of September there were 5,385 large transport airplanes registered in this country, of which 1,850 were 20 or more years old. Nine were more than 35 years old: three DC-8's, three 707's, and three 720's.

For the whole fleet, the average age was 15.82 years.

Wiring, according to plane manufacturers, is supposed to be good for the life of the plane. And several experts said today that any work on planes could damage wiring, even, for example, installing smoke detectors and fire suppression systems, as is now being done on thousands of planes in response to the Valujet crash of May 1996.

Also today, William Crow, a senior F.A.A. maintenance expert, said that he would hesitate to implement improvements in planes before the cause of this crash is clear.

''It's very important we stand firmly in our position as given to us in the regulations,'' he said. ''We should not blow in the wind one direction, or blow in the wind the other direction. We should not take action until such time as we are absolutely sure of probable cause.''

''We really have no probable cause,'' he said. ''We have ideas.'' Acting without the proper information, he said, ''You may do exactly the wrong thing.''

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A version of this article appears in print on , Section B, Page 6 of the National edition with the headline: 747 Fleet's Age at Issue During Flight 800 Hearing. Order Reprints | Today’s Paper | Subscribe